The present invention relates to an improved vacuum bag, to a method for producing the improved bag, and to a vacuum bag molding process employing the improved bag. The present invention further relates to a composite article produced by the process.
The vacuum bag technique is well known in the art. The technique is widely used in the manufacture of high-strength, light-weight and high-temperature composite articles, in particular reinforced plastic composites. Known vacuum bag processes can be divided into two main types: wet lay-up and dry lay-up. The wet lay-up process is less preferable because it requires more resin and more assembly time, and is therefore more expensive. The dry lay-up, or prepreg, process employs fibrous reinforcement materials preimpregnated with resin and partially cured.
The prepreg process can in turn be divided into two types: the bleed method and the no-bleed method. The bleed method requires extraction or displacement of excess resin during the molding and curing process, in order to produce the desired resin/reinforcement ratio and thus part strength. The bleed method, like the wet lay-up method, wastes resin and time and is therefore less preferable. The no-bleed method employs prepregs having a resin content near or at the desired net resin content. Since there is no resin displacement, there is no need for the use of bleeder materials or barrier materials to prevent resin from reaching and plugging the vacuum lines in the vacuum bag. The need for breather materials to distribute air and volatiles escaping from the prepreg during the curing process is also eliminated. Accordingly, the total cost reduction associated with the prepreg method of vacuum bag molding is substantial both in terms of materials and labor.
All of the presently known vacuum bag molding methods, however, employ vacuum bags comprised of smooth films or sheet materials. Use of smooth vacuum bags has certain disadvantages. No specific provision is made in the film for migration of the displaced air inside the vacuum bag to the vacuum ports. Residual air pockets between the vacuum bag and the composite article enclosed therein, if not removed, will cause defects in the composite article, resulting in rejection of the part. Wrinkles that form in the vacuum bag during the evacuation process, especially if the part is irregularly shaped, can be transferred to the surface of the part during curing, resulting in a defective part.
In order to reduce the tendency of smooth vacuum bags to trap air, the evacuation process must be slowed down. This reduces production rates and increases costs. In addition, air pockets and wrinkles that do form must be manually displaced during the evacuation process. Such manual smoothing reduces the production rate. Moreover, manual smoothing can result in perforation of the vacuum bag, or seam separation, which will produce a defective part and furthermore require expenditure of additional time to locate and repair the perforation or separated seam. Finally, production of irregularly shaped parts often entails the use of special vacuum bags corresponding to the shape of the part. Such bags are more costly to produce, and are more susceptible to failure due to puncture or seam separation.
In known prepreg processes, the prepregs are maintained at a low temperature in a refrigerator until use, in order to retard curing which would take place at ambient temperature. As each prepreg is stacked to build up the lay-up and subjected to the slow manual smoothing process, however, the temperatures of the prepregs rise. Consequently, by the time the lay-up has been completely assembled and is ready for application of vacuum and subsequent cure, a temperature gradient, and thus a gradient in curing rates, exists between the first and last prepregs. This gradient results in nonuniformities in the bonding between the prepreg layers of the lay-up. The required manual smoothing in addition tends to disrupt the alignment of the reinforcements (e.g., fibers) within the prepregs, adversely affecting the strength of the composite article.
A clear need exists, therefore, for an improved vacuum bag and vacuum bag molding process, in particular a prepreg process, which avoid the foregoing problems, and which in particular allow evacuation of air from the vacuum bag without the formation of air pockets and wrinkles. A need also exists for an improved composite article produced by the vacuum bag molding process, which avoids the foregoing problems.